A conventional example of a liquid crystal display unit in which each pixel includes subpixels is disclosed in U.S. Pat. No. 4,840,460 (first conventional example). FIG. 11 (a) shows an equivalent circuit of the example. In the figure, 101 and 102 are gate wirings; 103 and 104 are source wirings; 105 is a thin-film transistor (TFT); and 106 is a common electrode. A pixel at the intersection of gate wiring 101 and source wiring 103 is indicated in the equivalent circuit. One pixel is divided into three subpixels. Control capacitors C.sub.c1, C.sub.c2 and C.sub.c3 are coupled in series with effective capacitors C.sub.Lc1, C.sub.LC2 and C.sub.LC3 respectively. Voltage applied to a liquid crystal layer of each subpixel element is determined by the ratio between the capacitance of effective capacitor C.sub.LCi and that of control capacitor Ci (where i=1, 2 and 3), when selection signals are provided to gate wiring 101 to turn on TFT 105 and voltage V is provided from source terminal 103. In other words, since this liquid crystal display unit includes subpixels with different operating voltage levels and displays by controlling the capacitance of the control capacitors of the subpixels, viewing angle properties are improved.
In the above-mentioned U.S. patent document, a specific structure for a subpixel using a divided subpixel electrode and control capacitor electrode is disclosed. FIG. 11 (b) is a cross-sectional view of the subpixel element in which a subpixel electrode 113 is divided into three sections and a control capacitor is formed by sandwiching an insulating film 114 between the subpixel electrode and a control capacitor electrode 115. The capacitance of the control capacitor is controlled at a preferable level by varying the area of control capacitor electrode 115 at each subpixel. The control capacitor electrode is coupled to the drain electrode of TFT by a terminal 116. Referring to the figure, 112 indicates a liquid crystal layer, forming an effective capacitor between each section of subpixel electrode 113 and a common electrode 111. The U.S. patent document also discloses a subpixel element in which a pixel electrode is not divided and an insulating film, having a thickness and a dielectric constant associated with one subpixel different from those associated with other subpixels, is formed between the pixel electrode and a liquid crystal layer so as to act as a control capacitor. However, in this structure, the area of the control capacitor is controlled by the area of the subpixel, so that it is realistically difficult to set the capacitance of the control capacitor at a preferable level. Since the structure shown in FIG. 11 (b) can be designed more freely than the structure having an undivided pixel electrode, the former is more preferable than the latter.
However, several problems regarding the structure shown in FIG. 11 (b) have been found. First, in order to achieve this structure, control capacitor electrode 115 and insulating film 114 have to be additionally formed, thus increasing manufacturing cost. In addition, by utilizing white and black gradations that are independent of viewing angle, a plurality of subpixels of one pixel is activated at these gradations, and a gradation display (gray scale display) that is independent of viewing angle is provided by the number of subpixels displaying white or black gradations, so that the number of subpixels has to be increased with the addition of gradation to a display. As a result, the driving voltage increases. The third problem of the liquid crystal unit having a subdivided pixel electrode in each subpixel element is that sections formed with no electrodes--such as gaps between subpixels--increase, thus reducing the pixel display area and lowering the brightness of a display screen, lowering contrast due to the leakage of light from the gaps between the subpixels, and slowing the speed of response because of deformed liquid crystal molecular orientation caused by an electric field, spread in a horizontal direction at the edges of subpixel electrodes.
In order to solve the above-noted third problems, U.S. Pat. No. 5,245,450 discloses a liquid crystal display unit shown in FIG. 12 (second conventional example). A control capacitor electrode 126 below an insulating film 125 and on a bottom substrate 127 in the figure is made of a transparent conductive film, and is also formed at gaps between four subpixels 124. A common electrode 122 is formed on the interior surface of a top substrate 121, and a liquid crystal layer 123 is sandwiched between two substrates. The problem mentioned above is solved by activating liquid crystal layer 123 with voltage applied between control capacitor electrode 126 and control electrode 122. However, compared with other conventional TFT liquid crystal display units, the unit is produced at a low rate since the transparent conductive film and insulating film 125 have to be additionally formed.
Furthermore, a display unit disclosed in Published Unexamined (Laid-Open) Japanese Patent Application No. Hei 5-289108 (third conventional example) also solves the above-described third problem. According to the plan view shown in FIG. 13, the gap between subpixel electrodes 133 and 134 and the peripheral section of subpixels are covered with a control capacitor electrode 135, a shielding electrode made of chrome or the like. In the figure, 131 is a gate wiring, and 132 is a source wiring. In this conventional example, control capacitor electrode 135 can be formed at the same time when a chrome film is formed as a shielding section 136 for a TFT section, so that the manufacturing processes are simple. Contrast is also at a preferable level since light does not leak from the gap between subpixel electrodes 133 and 134. However, the brightness of the display decreases due to the complete shielding of light at the gap between subpixel electrodes 133 and 134.